The invention relates to a method and apparatus for producing crimped yarns composed of continuous synthetic fiber-forming polymeric filaments, especially polyethylene terephthalate filaments, wherein the yarn is guided between a first and a second set of feed and draw rolls or similar independently driven delivery systems, so that the yarn passes with overfeed through a texturing nozzle operating according to the air-jet or air-bulking principle to produce a large number of random loops or crimps in the individual filaments. Other than the selection of particular filaments, the type and degree of texturizing or bulking depends largely upon the amount of twist in the initial yarn and the amount of overfeed through the texturizing jet. The resulting texturized yarn is then directly spooled, usually under a high uniform tension. The resulting yarn product is characterized by a poor linear stabiity and very high boiling shrinkage values.
A general description of the air-jet texturizing process can be found in such texts as "Woven Stretch and Textured Fabrics", by B. L. Hathorne, Interscience Publishers, John Wiley & Sons, N.Y., Chapter 6, pages 104-117 (1964). More particularly, attention is directed to the Breen patents and especially U.S. Pat. Nos. 2,783,609 and 2,852,906 which are directed to the yarn product as well as the process and apparatus. The yarns produced in this way are especially characterized by the presence of many ring-like or crunodal loops irregularly spaced along the yarn surface although there is usually a substantial number of loops or crimps formed internally of the yarn as well.
A variation of this air-jet texturizing process has been disclosed by Field in U.S. Pat. No. 2,962,794 for the purpose of producing a jet texturized yarn which is highly bulky and which exhibits an extremely high extensibility under very slight tension, e.g. an extensibility of at least one fifth again and preferably one half again the unextended length of the yarn with recovery to at most the arithmetic mean of the extended and unextended lengths. This process and the resulting yarn product differ from the earlier Breen process and its product in that a relatively low twist yarn is subjected to relatively high overfeed in order to produce by the jet texturization a large number of so-called "meta-stable loops" which remain in the yarn at about zero tension but which are removed under a light to moderate tension. Field teaches a heat setting of these meta-stable loops into the yarn such that the end product is highly extensible due to the large extension and contraction of the preset loops.
From the Breen process and other previously known processes of this kind, i.e. for the air-jet texturizing of a continuous multifilament yarn, it has become evident that the texturized or bulked yarn product has only a slight or insubstantial stability. In order to measure the amount of stability of the yarn, it is usually determined by using a sample which has an initial length of one meter under a base load of about 1/100 grams per denier, subjecting this sample to a load of 1/3 grams per denier for 30 seconds and then, after relieving the texturized yarn again to the base load of 1/100 grams per denier for a period of 30 seconds, measuring the length of the yarn. The "instability" can be read immediately from a centimeter measuring stick as a percentage of the original length of the texturized yarn. Thus, the instability is measured as the percentage increase in the standard yarn length of one meter after a specified load applied for a specified time has been removed and the yarn permitted to return to its base load for a similar specified period of time.
Up to the present time, it has not been possible to achieve instabilities of less than 1% with such air-jet texturized yarns. Moreover, it has not been possible with such yarns to achieve smaller shrinkage values in connection with boiling. Hitherto, boiling shrinkage values have been more than 4%. Also, it has been previously necessary to respool the texturized yarn once again, first because the yarn spool produced directly in the texturizing operation exhibits such poor run-off properties that it is impossible to introduce these spools directly onto a knitting or weaving machine. In particular, frequent thread breakage is caused and represents a serious problem in using even tightly wound initial spools which have not been rewound. Likewise, respooling has been deemed necessary in order to produce low-shrinkage dyeing spools which, as is known, may only have slight thread or yarn tensions. Also, flat webs or sheets as finished articles produced from the known jet-texturized or air-bulked yarns are not only disadvantageous with respect to their inherent instability and their large boiling shrinkage, but are also disadvantageous due to a very noticeable "burr effect" which becomes apparent from the fact that the surfaces of the flat articles adhere to each other.
Another disadvantage of yarns produced according to the conventional air-jet texturing method is the nonuniformity of the crimping or looping effect. Previously, this lack of uniformity could only be partly reduced by providing the longest possible free running length of the yarn just before the final winding or spooling step.